1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly to a zoom lens system suitable for use in a photographing optical system for a digital still camera using a solid image pickup element such as a CCD.
2. Related Background Art
In recent years, attention has been given to a so-called digital still camera for photographing a still image using a solid-state image pickup element such as a CCD. In order to reduce an entire size of the camera, an optical system having an extremely short entire lens length has been required for a zoom lens serving as a photographing optical system of the digital still camera.
In view of a characteristic of the still image, it is desirable that the digital still camera have a wide field angle. Also, the digital still camera is required to have higher optical performance than that of a video camera for moving picture photographing.
According to, for example, Examined Japanese Patent Application Publication No. H06-066008 (corresponding to U.S. Pat. No. 4,662,723), a zoom lens has been proposed as a zoom lens of a type in which a zoom ratio is about 2.5 to 3, a wide-angle range is ensured, an intensity is high, and high performance is obtained. This zoom lens includes two lens units, that is, a lens unit having a negative refractive power and a lens unit having a positive refractive power. Zooming is performed by changing an interval between the respective lens units.
According to, for example, Examined Japanese Patent Application Publication No. H07-052256 (corresponding to U.S. Pat. No. 4,733,952), there is an example of a zoom lens including three lens units, that is, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a positive refractive power. An interval between the second lens unit and the third lens unit is changed during zooming from a wide-angle end to a telephoto end.
According to U.S. Pat. No. 5,434,710, there is an example of a zoom lens which includes three lens units, that is, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a positive refractive power and in which an interval between the second lens unit and the third lens unit reduces during zooming from the wide-angle end to the telephoto end.
According to Japanese Patent Application Laid-Open No. H10-104520, there is an example of a zoom lens of a super wide-angle type which includes four lens units, that is, a lens unit having a negative refractive power, a lens unit having a positive refractive power, a lens unit having a positive refractive power, and a lens unit having a positive refractive power.
According to Japanese Patent Application Laid-Open No. H11-084242 (corresponding to U.S. Pat. No. 6,191,896), there has been proposed a zoom lens of a zoom type which includes four lens units, that is, a lens unit having a negative refractive power, a lens unit having a positive refractive power, a lens unit having a positive refractive power, and a lens unit having a positive refractive power. The zoom lens is compact and has a zoom ratio of about 3. A variation in exit pupil position during zooming is relatively small.
In the case of a zoom lens of a conventional type in which a lens unit having a negative refractive power precedes other lens units, a first lens unit is generally composed of three or more lenses. In order to obtain a compact structure, there have been proposed various zoom lenses, in each of which the first lens unit is composed of two lenses (negative lens and positive lens) in which an aspherical surface is provided for the negative lens.
Examples in which the first lens unit is composed of two lenses (negative lens and positive lens) and an aspherical lens is used for the positive lens are disclosed in Japanese Patent Application Laid-Open No. H11-084243 (corresponding to U.S. Pat. No. 6,233,099), Japanese Patent Application Laid-Open No. 2001-147370, U.S. Pat. No. 5,663,836, U.S. Pat. No. 5,872,660, and U.S. Pat. No. 6,025,961.
The two-unit zoom lens as disclosed in Examined Japanese Patent Application Publication No. H06-066008 includes a large number of lenses and is insufficient in view of compactness. A variation in exit pupil position during zooming is large. Though there is no problem when the zoom lens is used for silver-halide film camera, when the zoom lens is used for a digital still camera including a solid-state image pickup element, design power allocation is restricted, so a size of the zoom lens is likely to increase.
As in the two-unit zoom lens described in Examined Japanese Patent Application Publication No. H06-066008, even in the zoom lens described in Examined Japanese Patent Application Publication No. H07-052256, a variation in exit pupil position during zooming is large. Therefore, the zoom lens described in Examined Japanese Patent Application Publication No. H07-052256 is inappropriate for a photographing optical system for digital still camera.
The zoom lens disclosed in U.S. Pat. No. 5,434,710 includes a large number of lenses, so there is a limitation to reduce a size thereof.
With respect to the zoom lens disclosed in Japanese Patent Application Laid-Open No. H10-104520, although only a zoom ratio of about 2.4 is obtained, the number of constituent lenses is large. In addition, the lens arrangement is made to realize the super wide angle, so sufficient compactness is not obtained.
The zoom lens disclosed in Japanese Patent Application Laid-Open No. H11-084242 puts a priority on a reduction in size thereof, so the number of constituent lenses is very small. Therefore, the zoom lens is suitable for use in a low-cost digital still camera having a small number of pixels. However, high performance suitable for a digital still camera having a high pixel density is not obtained.
An uneven thickness ratio between a central portion and a peripheral portion of the negative lens in the first lens unit is large. Therefore, when an aspherical lens is used as the negative lens, there is a problem in that a manufacturing cost increases.